Related Field
The subject matter discussed herein relates generally to overvoltage protection devices or circuits and methods, and particularly to overvoltage protection devices and methods for electrical or electronic medical devices.
Related Background
Active electrical devices intended for medical and veterinary applications usually serve specific purpose. ECG monitors are used to obtain an electrocardiogram, hemodynamic monitors employing measurement of electrical impedance apply an electrical auxiliary current, electrocautery knifes and other electrosurgical devices are used in the operating room, electrical pulse generators emit electrical stimuli for stimulation of cardiac tissue or nerves, and cardiac defibrillators apply high voltage pulses towards the heart to overcome fibrillation. The chance that more than one such device is used on the same patient at the same time is high. It is therefore important to shield each electrical or electronic medical device from damage resulting from the electrical energy applied by another such device (“protection requirement”). Electrical medical devices should also be designed not to absorb the energy applied by another device for treatment of the patient or animal (“energy reduction requirement”).
The compatibility between electrical devices applied simultaneously to a patient or animal is of particular of importance upon applying a defibrillation pulse to the patient. Defibrillation pulses with voltage amplitude reaching or exceeding several hundreds and even thousands of volts (“overvoltage”) create a challenge for other electrical medical devices used at the same time. The challenge is greater for devices emitting electrical pulses (pacemakers) or signal waveforms (patient auxiliary currents). Such devices have a low impedance interface towards the patient (or animal). For instance, the two states in which a pacemaker usually exhibits low impedance of the patient line interfaces is during the stimulation pulse and the discharge period immediately following the stimulation pulse. While pacemakers require a patient line interface impedance in the range of a few ohms, bio-impedance based monitors typically operate with impedances of several hundreds of ohms up to about 1 kΩ. In summary, any impedance added in the interface for protective reasons might compromise the function of the emitting device.
A simple protection against defibrillation pulses can be achieved by employing a transient voltage suppressor diode (TVS) in conjunction with a “protection resistor” in series with the patient (human or animal) line interface, and applied to each patient line interface. In order to achieve a suitable defibrillation energy reduction, the value of a defibrillation protection resistor cannot be small. Instead, a relative high-ohmic resistor is required. Thus, this simple defibrillation protection approach cannot be used in active electrical devices such as cardiac pacemakers and biompedance-based monitors which require a low impedance interface.
A commonly used approach to switch between a low impedance patient interface during normal operation and a high impedance interface upon occurrence of overvoltage is the employment of depletion mode metal-oxide-semiconductor field-effect transistors (MOSFETs) in line with the patient interface. These devices act as current limiters, since a MOSFET in off-state allows only minimal current passing through.
Many known overvoltage protection circuits for medical devices have in common an element in series with the patient line interface to bias a MOSFET transistor so that it can impede current sufficiently. This element may comprise a resistor or another transistor. In either case, this element increases the impedance of the patient line interface during normal operation by a smaller amount employing a transistor and by a larger amount using a resistor. In addition to this drawback, some solutions employ a relatively complicated MOSFET biasing technique, which is dependent on biasing voltages generated by the protected device itself. This may lead to compromising situations because the protection is relying on an internal voltage, the level of which can become questionable due to a fault or battery exhaustion.
Some protection circuits have capacitors in series with each patient line interface. The capacitors change the shape of incoming transients in such a way that only a fraction of the initial overvoltage transient energy is passing through. The remaining voltage is clamped by voltage limiting devices such as Zener diodes. The combination of a capacitor and a Zener-diode repels the incoming energy. A potential shortcoming of such a circuit is related to the capacitors. A reliable protection against high voltage transients requires the use of special high voltage capacitors which are not of the ordinary type and, accordingly, most likely expensive and/or hard to get. Another potential shortcoming is that the sensitivity of the protection circuit is dependent on the value of applied capacitors. Such protection circuits may be trimmed in view of the expected overvoltage transients, which is not a desired property of an overvoltage protection circuit. Moreover, the required capacitors connected in series with each patient interface line establish an impedance, the value of which, unfortunately, is also dependent on the capacitor value.